ABSTRACT: The influence of solar radiation and hydrogen peroxide on induction of lysogens, and the resulting effect on bacteriophage production and bacterial mortality was investigated for coastal and oceanic marine bacterial communities at 6 stations in
the western Gulf of Mexico. The percentage of lysogenic cells induced by mitomycin C was also determined. Solar radiation and hydrogen peroxide were not as effective as mitomycin C at inducing phage production. The burst size of cells induced by mitomycin
C was estimated by transmission electron microscopy, assuming that cells completely filled with viral particles were on the verge of bursting. The smallest estimates of burst size were associated with oligotrophic oceanic stations and ranged from 15 to
28 viruses produced per lytic event, while in more productive coastal waters the estimated burst sizes ranged from 33 to 64. The mitomycin C-induced phage production and burst size were used to estimate the number of lysogenic bacterial cells. On average,
the percentage of inducible lysogens was higher at offshore (1.5 to 11.4%) than at coastal (0.8 to 2.2%) stations. However, with the exception of 1 station, less than 5% of the bacteria could be induced to produce phage, suggesting that lysogens only
occasionally comprised a significant component of these bacterial communities. The proportion of lysogens that could be induced by sunlight, relative to those that could be induced by mitomycin C, was lower at oceanic than coastal stations. This implies
that prophages in optically transparent offshore waters were more resistant to induction by solar radiation, or that most lysogens that could be triggered by sunlight were already induced. Based on a steady-state model, induction of lysogenic bacteria by
solar radiation or hydrogen peroxide could result in between 0 and 3.5% or 0.9 and 3.4% of the total bacterial mortality, respectively. Our results imply that solar radiation and hydrogen peroxide induced lysogenic phage production were not an important
source of phage production or bacterial mortality in offshore or coastal waters of the western Gulf of Mexico.